In semiconductor processing technology, a desired goal is the uniformity of process results. One of these process steps is the deposition of material upon semiconductor substrates from gases. This is called chemical vapor deposition. Semiconductor substrates are placed into a reactor on a carrier or susceptor, which is then heated by induction and/or high intensity light radiation to high temperatures, typically 1000.degree. C.-1300.degree. C. Gases are passed through the reactor and the deposition process occurs by a chemical reaction in the gases. The reacting gases deposit material on the substrates.
One of these chemical vapor deposition processes, epitaxial silicon deposition, is the formation of a single crystal film on top of a single crystal substrate. Satisfactory epitaxial silicon deposition requires control of crystal surface defects, undesired impurities, and uniformity of both thickness and doping concentration of the epitaxial silicon. The doping concentration in the epitaxial layer determines the resistivity or conductivity of the layer. Variations in the thickness of the deposited epitaxial layer may occur on substrates at different locations on the carrier or susceptor and even on a single substrate. The uniformity of the epitaxial layer is controlled by the specific combination of the mainstream gas flow rate, the specific geometry of the reactor enclosure, including the shape and location of the gas inlet, and the temperature uniformity of the susceptor.
On the other hand, the average resistivity is controlled by the average dopant concentration in the gas stream and the average temperature. Since the incorporation of dopants into the epitaxial layer is more strongly related to the local temperature of a susceptor region and the overlying substrate than the local growth rate of the epitaxial silicon on the overlying substrate, localized variations in temperature have a larger effect upon resistivity rather than thickness. The result is that resistivity uniformity is primarily controlled by the local temperature values.
In a semiconductor manufacturing operation, the epitaxial layer thickness is typically controlled by optimizing all the variables noted above. The nonuniform temperature profiles are compensated for by making mechanical and flow rate adjustments. Sometimes nonuniform temperature variations are purposely introduced to achieve uniform deposition thickness, which compromises the resulting resistivity uniformity. Heretofore, there has been difficulty in achieving uniform resistivity and thickness simultaneously.
Two main classes of reactors are used for chemical vapor deposition, the "hot wall" reactor and the "cold wall" reactor. In a hot wall reactor, all parts of the reactor are heated so that the reactor walls are at the same temperature as the susceptor or carrier, which bears the semiconductor substrates. In a cold wall reactor, the energy for heating the system is directed toward the susceptor or carrier. The reactor walls are thus cooler than the susceptor or carrier.
In the class of cold wall reactors, there are three types of reactors--the vertical reactor, the horizontal slab reactor and the cylinder reactor. The present invention relates to the first of these reactor types.
With the problems above, it has been widely believed that the vertical reactor, which has been in commercial existence for more than fifteen years, is capable of no better than a .+-.5%-10% thickness uniformity and a .+-.8%-15% resistivity uniformity for silicon epitaxial deposition.